Appendix II. 821 the only requirement in a true sphere. After the ball is made as perfect as possible with the usual turning tools, it is finished with the tool B, made from steel tube, which is rocked to and fro as the lathe revolves, the ball being often removed. P. 274. Jigs.—These appliances may be constructed so as to hold work for other operations besides drilling bolt holes, and Fig, 791 shews two arrangements—A being a special chuck for holding a dome cover while turning, and B a vice for supporting a loose collar for boring. Cutting Wheel Teeth,—The formation of the teeth of wheels, either on paper or in the workshop, has been mentioned at various places in this book as follows :— Kind of Tooth. Patterns. Moulds. Cutting in Metal. Describing Teeth Curves. Spur p. 60 P- 31 pp. 175 & 1 80 pp. 510 & 517 Bevel p. 60 P- 31 pp. 256 & 753 P- 5*9 Worm P- 5* p. 10 p. 274 — A few more words on this very important subject will not be out of place. Spur-wheel Teeth.—A milling cutter is always used to remove the interspaces, but the * blank' to be cut may be mounted in one of various ways. On p. 180, the dividing heads are used for support, but this method is only suitable for small diameters. When wheels of large diameter form the regular work of a shop, special machines are adopted; in many cases so constructed as to- automatically rotate the wheel through the pitch arc after every cut. Such a machine is shewn in Fig. 792, elaborate but effective, where A is the cutter, B the work, c the dividing wheel, and D a. sector for tilting the table to suit bevel gears. Yet another method, shewn in Swasey's machine, Fig. 793, is based on the fact that if a number of wheels be cut so as to each gear with a given rack, they will all gear the one with the other.